The present invention relates to an objective lens used for conducting recording and/or reproducing of an information recording medium in an optical pickup apparatus, and to an optical pickup apparatus.
For an optical pickup apparatus capable of conducting recording and/or reproducing of information for a plurality of information recording media (for example, optical disks) each having different recording density and a different thickness of a transparent base board, such as CD and DVD, it has been demanded that downsizing and cost reduction of the apparatus are realized by using at least an objective lens in common, and various objective lenses and various optical pickup apparatuses for that demand have been suggested.
For example, there has been known an objective lens wherein ring-shaped notches are provided on one surface of the objective lens as specific ring-shaped zones to constitute the surface with a plurality of divided surfaces (for example, three divided surfaces), and a part (for example, a first divided surface closer to the optical axis) of the plural divided surfaces is made to be used for recording and/or reproducing of information for two types of information recording media each having different recording density and a different thickness of a transparent base board, a part (for example, a second divided surface that is adjacent to the first divided surface) of the remaining divided surfaces is made to be used for recording and/or reproducing of information for the information recording medium on one side (for example, the information recording medium having a smaller necessary numerical aperture), and the rest (for example, a third divided surface that is adjacent to the second divided surface) is made to be used for recording and/or reproducing of information for the information recording medium on the other side (for example, the information recording medium having a larger necessary numerical aperture). As an example of the foregoing, there may be given one described in TOKKAIHEI No. 11-96585. However, erroneous detection has sometimes been caused for focus signals on the objective lens on which ring-shaped notches are provided as stated above. For example, in the case of recording or reproducing of DVD through the second divided surface that is corrected in terms of spherical aberration for CD among three divided surfaces provided on one surface of the objective lens, a light flux which has passed through the second divided surface has sometimes been converged on a sensor through defocusing, to cause erroneous detection for focus signals.
In TOKKAIHEI No. 10-283668, there is described an optical pickup apparatus wherein a ring lens of a hologram type is used, and with regard to a hologram portion, light having a wavelength of 650 nm for DVD is used as zero-order light, while, with regard to light having a wavelength of 780 nm for CD, one-order diffracted light is used.
However, in the case of one wherein divided surfaces are provided on an objective lens as stated above, and one of the divided surfaces is made to be a hologram or a diffractive surface for which zero-order light is used for a wavelength on one side and primary light is used for a wavelength on the other side, the diffraction efficiency is lowered, and quantity of light coming from a light source cannot be utilized sufficiently, resulting in erroneous detection that is sometimes caused for focus signals.
In TOKKAI No. 2000-81566, there is described an objective lens wherein a diffractive structure is formed on a lens surface in a pattern of ring-shaped zones whose centers are on an optical axis, and the diffractive structure is made to have wavelength-dependency so that diffracted light with the same order number by at least two light fluxes each having a different wavelength may form an excellent wave front respectively for two types of optical disks each having a different thickness of a protective layer.
However, since the diffractive structure is formed on the objective lens equipped with diffractive structure on the entire surface of the lens surface as stated in the official gazette mentioned above so that the diffracted light with the same order number appropriate for light fluxes with at least two wavelengths may be obtained, there has been a problem that efficiency of the objective lens for using quantity of light from a light source is lower than that of an objective lens composed of a refraction surface on which no diffractive structure is provided. Further, since the number of diffractive ring-shaped zones is large, productivity for a metal mold for forming a lens is lowered, and production cost for a metal mold is enhanced, which has also been a problem. When recording information on an optical information recording medium, in particular, the recording requires greater quantity of light, compared with reproducing, and therefore, the problem of the fall of efficiency for using quantity of light is serious.
Further, when using a plastic lens, a change in aberration caused by temperature fluctuation has also been a problem.
In view of the problems in the prior art stated above, an object of the invention is to provide an optical pickup apparatus wherein quantity of light coming from a light source can be used fully, erroneous detection of light from an information recording medium is not caused, and an increase in production cost can be controlled, and to provide an objective lens to be used for the optical pickup apparatus. A further object is to provide an optical pickup apparatus capable of providing sufficient quantity of light when recording information on an optical information recording medium and to provide an objective lens to be used for the optical pickup apparatus stated above. Still another object is to provide an optical pickup apparatus that can control a change in aberration even when ambient temperature is changed, and to provide an objective lens to be used for the optical pickup apparatus mentioned above.
The objects stated above can be attained by the following structures.
(1) An optical pickup apparatus for conducting reproducing or recording information of at least two kinds of optical information recording media, comprises:
a first light source to emit first light flux having a first wavelength to conduct reproducing or recording information of a first optical information recording medium having a first transparent base board;
a second light source to emit second light flux having a second wavelength to conduct reproducing or recording information of a second optical information recording medium having a second transparent base board;
an optical converging system having at least one optical element; and
a photo detector to receive and detect light transmitting through or reflecting from a first information recording surface of the first optical information recording medium or light transmitting through or reflecting from a second information recording surface of the second optical information recording medium;
wherein the first wavelength is different from the second wavelength and the thickness of the first transparent base board is different from that of the second transparent base board,
wherein the optical element comprises a first region including an optical axis and a second region adjoining the first region and locating at the outside of the first region, the first region is a refractive surface and the second region is a diffractive surface,
wherein when the first light flux passes through the second region of the optical element, the light amount of n-th order diffracted ray of the first light flux is larger than that of any other order diffracted ray of the first light flux, and when the second light flux passes through the second region of the optical element, the light amount of n-th order diffracted ray of the second light flux is larger than that of any other order diffracted ray of the second light flux, where n is an integer except 0,
wherein the optical converging system converges the first light flux having passed through the first region and the n-th order diffracted ray of the first light flux having passed through the second region through the first transparent base board onto the first information recording surface so as to conduct reproducing or recording information of the first optical information recording medium, and the optical converging system converges the second light flux having passed through the first region through the second transparent base board onto the second information recording surface so as to conduct reproducing or recording information of the second optical information recording medium.
By virtue of the structure above, efficiency for using quantity of light is better than that in the occasion where the diffractive structure is provided on the entire surface, and it is possible to shorten man-hour for processing on a metal mold for forming a lens and to reduce production cost for the metal mold, because the number of diffractive ring-shaped zones is small. Further, erroneous detection of light from an information recording medium can also be prevented.
(2) In the optical pickup apparatus of (1), the optical element comprises a third region adjoining the second region and locating at the outside of the second region and the third region is a refractive surface, and
the optical converging system converges the first light flux having passed through the first region, the n-th order diffracted ray of the first light flux having passed through the second region and the first light flux having passed through the third region through the first transparent base board onto the first information recording surface so as to conduct reproducing or recording information of the first optical information recording medium, and the optical converging system converges the second light flux having passed through the first region and the n-th order diffracted ray of the second light flux having passed through the second region through the second transparent base board onto the second information recording surface so as to conduct reproducing or recording information of the second optical information recording medium.
By virtue of the structure above, efficiency for using quantity of light is further improved by reducing an area of the diffraction portion more, and it is possible to shorten man-hour for processing on a metal mold for forming a lens and to reduce production cost for the metal mold, because the number of diffractive ring-shaped zones is small.
(3) In the optical pickup apparatus of (1), the optical element comprises a third region adjoining the second region and locating at the outside of the second region and the third region is a diffractive surface,
wherein when the first light flux passes through the third region of the optical element, the light amount of m-th order diffracted ray of the first light flux is larger than that of any other order diffracted ray of the first light flux, and when the second light flux passes through the third region of the optical element, the light amount of m-th order diffracted ray of the second light flux is larger than that of any other order diffracted ray of the second light flux, where m is an integer except 0 and is equal to or different from n, and
wherein the optical converging system converges the first light flux having passed through the first region, the n-th order diffracted ray of the first light flux having passed through the second region and the m-th order diffracted ray of the first light flux having passed through the third region through the first transparent base board onto the first information recording surface so as to conduct reproducing or recording information of the first optical information recording medium, and the optical converging system converges the second light flux having passed through the first region and the n-th order diffracted ray of the second light flux having passed through the second region hrough the second transparent base board onto the second information recording surface so as to conduct reproducing or recording information of the second optical information recording medium.
By virtue of the structure above, efficiency for using quantity of light is improved, production cost of a metal mold is reduced, and it is possible to control a change in aberration caused by temperature fluctuation. In addition, when conducting recording/reproducing of information on an optical information recording medium whose necessary numerical aperture is small, control of a light flux for making a light flux on the portion greater than the necessary numerical aperture to be a flare is more easy. Namely, flare control becomes easy. Thus, erroneous detection of light from an information recording medium can further be prevented.
(4) In the optical pickup apparatus of (1), the optical converging system comprises an objective lens and the second region extends to the maximum numerical aperture of the objective lens at an optical information recording medium side when the first light flux is used.
By virtue of the structure above, efficiency for using quantity of light is improved, production cost of a metal mold can be reduced, and control of a light flux for making a light flux on the portion greater than the necessary numerical aperture to be a flare is more easy when conducting recording/reproducing of information on an optical information recording medium whose necessary numerical aperture is small. Namely, flare control becomes easy. Thus, erroneous detection of light from an information recording medium can further be prevented.
(5) An objective lens for use in an optical pickup apparatus for conducting reproducing or recording information of at least two kinds of optical information recording media, comprises:
a first region including an optical axis; and
a second region adjoining the first region and locating at the outside of the first region;
wherein the first region is a refractive surface and the second region is a diffractive surface,
when a first light flux having a first wavelength to conduct reproducing or recording information of a first optical information recording medium having a first transparent base board passes through the second region of the objective lens, the light amount of n-th order diffracted ray of the first light flux is larger than that of any other order diffracted ray of the first light flux, and when a second light flux having a second wavelength to conduct reproducing or recording information of a second optical information recording medium passes through the second region of the optical element, the light amount of n-th order diffracted ray of the second light flux is larger than that of any other order diffracted ray of the second light flux, where n is an integer except 0,
wherein the objective lens converges the first light flux having passed through the first region and the n-th order diffracted ray of the first light flux having passed through the second region through the first transparent base board onto a first information recording surface of the first optical information recording medium so as to conduct reproducing or recording information of the first optical information recording medium, and the objective lens converges the second light flux having passed through the first region through the second transparent base board onto a second information recording surface of the second optical information recording medium so as to conduct reproducing or recording information of the second optical information recording medium, and
wherein the first wavelength is different from the second wavelength and the thickness of the first transparent base board is different from that of the second transparent base board.
By virtue of the structure above, efficiency for using quantity of light is better than that in the occasion where the diffractive structure is provided on the entire surface, and it is possible to shorten man-hour for processing on a metal mold for forming a lens and to reduce production cost for the metal mold, because the number of diffractive ring-shaped zones is small. Further, erroneous detection of light from an information recording medium can also be prevented.
(6) An optical information recording medium recording or reproducing apparatus for conducting reproducing recording information of at least two kinds of optical information recording media, comprising:
the optical pickup apparatus recited in (1).
By virtue of the structure above, efficiency for using quantity of light is better than that in the occasion where the diffractive structure is provided on the entire surface, and it is possible to shorten man-hour for processing on a metal mold for forming a lens and to reduce production cost for the metal mold, because the number of diffractive ring-shaped zones is small. Further, erroneous detection of light from an information recording medium can also be prevented.
In the embodiment of an objective lens used for an optical pickup apparatus in the invention, there is provided a lens surface that is equipped with areas each having no diffractive structure arranged on both sides of an area having thereon a diffractive structure in the direction toward the outside from the optical axis, and the diffractive structure is characterized to take a form that gives the same order number (excluding 0-order light) to light fluxes with at least two wavelengths (xcex1 and xcex2) wherein the order number of the diffracted light generates the maximum quantity of diffracted light. Here, xe2x80x9cthe same order numberxe2x80x9dxe2x80x9d means that a number including a sign of xe2x80x9c+xe2x80x9d or xe2x80x9cxe2x88x92xe2x80x9d is the same. It may be preferable that the same order number is to be xe2x80x9c+1xe2x80x9d. Incidentally, the condition that the sign of the diffraction order number is xe2x80x9c+xe2x80x9d means that the diffracted light ray is converged more than a zero order light ray.
The position where the area equipped with the above-mentioned diffractive structure is provided is determined by a wavelength of a light source used in an optical pickup apparatus that employs the objective lens stated above, a thickness of a transparent base board of an information recording medium, and density for recording information on the information recording medium, and is decided in accordance with the necessary numerical aperture of an objective lens determined for each of the information recording media.
For example, it is preferable that numerical apertures needed for objective lenses for reproducing or recording two information recording media in the optical pickup apparatus are different each other, and an area equipped with the diffractive structure is set in the vicinity of the smaller numerical aperture among the aforesaid numerical apertures. In virtue of the foregoing, it is possible to conduct light-converging for an appropriate spot diameter having the diffraction limit power for an information recording medium having a smaller necessary numerical aperture, and thereby to make the area that is not used usually for the information recording medium having a larger necessary numerical aperture, and might cause erroneous detection of focus signals by converging light through defocusing to be capable of contributing to image forming, on the contrary, and it is possible to make a loss of quantity of light to be less than the diffractive structure that causes the loss of quantity of light for the different wavelength, by providing areas each having no diffractive structure on areas by which the area having a diffractive structure is sandwiched. Therefore, it is possible to increase efficiency of using quantity of light from a light source that contributes to recording or reproducing of information, and to improve efficiency of an optical pickup apparatus by preventing erroneous detection of focus signals.
By providing areas each having no diffractive structure so that they sandwich the area having a diffractive structure as stated above, the efficiency of using quantity of light is improved more, compared with an occasion where diffractive structure is provided on the entire surface, thus, processing man-hour for a metal mold for forming lenses is shortened because the number of ring-shaped zones for diffraction is small, and fabrication cost for a metal mold can be reduced.
When an area that is not equipped with the diffractive structure is formed as a refraction surface on an area including an optical axis on the lens surface, the efficiency of using quantity of light is more improved.
An embodiment of the objective lens for the optical pickup apparatus in the invention is characterized in that the optical pickup apparatus has a first light source having a wavelength of xcex1 and a second light source having a wavelength of xcex2 (xcex1 less than xcex2), the first light source emits a first light flux for reproducing or recording information for the first optical information recording medium having a transparent base board whose thickness is t1, the second light source emits a second light flux for reproducing or recording information for the second optical information recording medium having a transparent base board whose thickness is t2, the aforesaid objective lens has an area that is provided, on at least one surface, with diffractive structure that is rotation-symmetrical with respect to an optical axis, and the following expressions are satisfied when NA1 represents necessary numerical aperture of the objective lens on the optical information recording medium side that is needed for recording and/or reproducing the first optical information recording medium with the first light source, NA2 (NA2 less than NA1) represents necessary numerical aperture of the objective lens on the optical information recording medium side that is needed for recording and/or reproducing the second optical information recording medium with the second light source, the objective lens has on at least one surface thereof an area provided with a diffractive structure that is rotation-symmetrical with respect to an optical axis, N-th order diffracted light (N is an integer other than zero) coming from the area provided with the diffractive structure is utilized when recording and/or reproducing the first optical information recording medium with the first light source, M-th order diffracted light (M=N) coming from the area provided with the diffractive structure is utilized when recording and/or reproducing the second optical information recording medium with the second light source, and when N-th order diffracted light coming from the circumference of a circle that is farthest from an optical axis on the area provided with the diffractive structure among the light flux from the first light source is converted into the light flux whose numerical aperture on the optical information recording medium side is NAH1, while when N-th order diffracted light coming from the circumference of a circle that is closest to an optical axis on the area provided with the diffractive structure among the light flux from the first light source is converted into the light flux whose numerical aperture on the optical information recording medium side is NAL1.
NAH1 less than NA1
(1/3) NA2 less than NAL1 less than NA2
In the system using a light source having a wavelength of either one of 655xc2x130 nm for DVD and using a light source having a wavelength of either one of 785xc2x130 nm for DC in the optical pickup apparatus of a DVD/CD interchangeable type, 0.45 less than NAH1 less than 0.56 and 0.3 less than NAL1 less than 0.45 are preferable. The more preferable is that the position of an area provided with a diffractive structure is the numerical aperture 0.3-0.5 of the objective lens on the image side, and the still more preferable is that the position of an area provided with a diffractive structure is the numerical aperture 0.35-0.47 of the objective lens on the image side.
Further, 0.05 less than (NAH1-NAL1) less than 0.20 is preferable, and 0.07 less than (NAH1-NAL1) less than 0.13 is more preferable.
As a concrete design, for example, in an optical pickup apparatus in which DVD/CD are interchangeable, for DVD, a region up to a numerical aperture necessary for DVD except a region provided with a diffractive structure, that is, a central region and a peripheral region are shaped a spherical surface in which spherical aberration is corrected very well. On the other hand, in order to correct spherical aberration deteriorated due to the different thickness of the transparent base board of CD so as to conduct converging a beam having an appropriate spot diameter provided with a diffraction limitation performance, for CD in which the thickness of a transparent base board is different from that of DVD, a region in the vicinity of a numerical aperture necessary for CD, that is, a middle region is shaped a basic aspherical surface having a different aspherical surface from the spherical surface in which spherical aberration is corrected very well for DVD. Further, since the spherical aberration for DVD is deteriorated with only the basic spherical surface, a diffractive structure is provided to the region of the basic spherical surface. As a result, a first order diffracted ray from the region contributes to an image formation for DVD, thereby correcting the spherical aberration. The diffractive structure is designed such that the first order diffracted ray from the region contributes to an image formation for CD.
It is naturally possible to design an objective lens of the invention with various applications, without being limited to the design of this kind. Incidentally, the optical pickup apparatus of a DVD/CD interchangeable type is an optical pickup apparatus wherein at least one of information recording and information reproducing is possible for at least one type of DVD, and at least one of information recording and information reproducing is possible for at least one type of CD. CDs in various types include CD-R, CD-RW, CD-Video and CD-ROM, for example, and DVD includes, for example, DVD-ROM, DVD-RAM, DVD-R and DVD-RW.
Further, N equals 1, and it is preferable that primary diffracted light is used for both of the first and second information recording media.
In the boundary between the area provided with the diffractive structure and the area that is closest to an optical axis and is not provided with diffractive structure, it is preferable that a phase slip of a wavefront caused when light with the wavelength xcex1 is transmitted through a transparent base board having the thickness of t1 is not more than xcex1/10 when the first light source and the first optical information recording medium are used, and the more preferable is xcex1/20 or less.
Further, it is preferable that a step portion is provided on the circumference of a circle on the area provided with the diffractive structure closest to an optical axis, and a depth of the step portion is set so that a difference between optical paths caused by the step portion on the boundary with a refracting interface is almost integral multiples of xcex1 and xcex2, and due to this, a phase slip caused by the step portion can be made to be almost zero by each wavelength of xcex1 and xcex2.
To be concrete, it is possible to make the diffractive structure to be recessed from the refracting interface or to be protruded from the refracting interface in terms of a form so that a step portion is provided on the circumference of a circle on the area provided with the diffractive structure closest to an optical axis and a depth of the step portion is within a range of 4 xcexcm-10 xcexcm.
On the other hand, it is possible to make a step portion to be provided also on the circumference of a circle on the area provided with the diffractive structure farthest from an optical axis, and it is possible to make the refracting interface that is farther from an optical axis to be recessed from the diffractive structure or to be protruded from the diffractive structure, on the step portion. This structure can make deterioration of wavefront aberration in temperature fluctuation to be small. Namely, it is possible to arrange so that a step portion is provided in the direction of an optical axis at the boundary between an area having thereon the diffractive structure and an area that is farthest from an optical axis and has no diffractive structure, and this step portion has a step of 1 xcexcm-10 xcexcm in the direction of an optical axis. When the step portion is 1 xcexcm or more, it is possible to enhance Strehl ratio on the second optical information recording medium (for example, CD) side, while, when it is 10 xcexcm or less, temperature characteristics about wavefront aberration are not deteriorated. Incidentally, the step portion provided in the direction of an optical axis may also be provided to be aslant to the direction of an optical axis of course, and even in this case, the step in the direction of an optical axis has only to be 1 xcexcm-10 xcexcm. In particular, the step that is in parallel with the direction of an optical axis is preferable.
The preferable optical pickup apparatus of the invention is provided with a first light source that emits a first light flux having wavelength of xcex1 for reproducing or recording of information for the first optical information recording medium with a transparent base board whose thickness is t1, a second light source that emits a second light flux having wavelength of xcex2 (xcex1 less than xcex2) for reproducing or recording of information for the second optical information recording medium with a transparent base board whose thickness is t2, an optical detector that detects light from the first and second information recording media, and with an objective lens wherein there is provided, in the direction toward the outside from the optical axis side, a lens surface equipped thereon with areas each having no diffractive structure in which an area having a diffractive structure is interposed, and the diffractive structure is in a form which makes at least two light fluxes each having a different wavelength (xcex1, xcex2) generate the maximum amount of diffracted light to be of the same order number each other (excluding 0 order).
Another preferable optical pickup apparatus of the invention is characterized in that there are provided a first light source that emits a first light flux having wavelength of xcex1 for reproducing or recording of information for the first optical information recording medium with a transparent base board whose thickness is t1, a second light source that emits a second light flux having wavelength of xcex2 (xcex1 less than xcex2) for reproducing or recording of information for the second optical information recording medium with a transparent base board whose thickness is t2, an optical detector that detects light from the first and second information recording media, and an objective lens having on at least one surface thereof an area provided with diffractive structure that is rotation-symmetrical with respect to an optical axis, and the following expressions are satisfied when NA1 represents necessary numerical aperture of the objective lens on the optical information recording medium side that is needed for recording and/or reproducing the first optical information recording medium with the first light source, NA2 (NA2 less than NA1) represents necessary numerical aperture of the objective lens on the optical information recording medium side that is needed for recording and/or reproducing the second optical information recording medium with the second light source, N-th order diffracted light (N is an integer other than zero) coming from the area provided with the diffractive structure is utilized when recording and/or reproducing the first optical information recording medium with the first light source, M-th order diffracted light (M=N) coming from the area provided with the diffractive structure is utilized when recording and/or reproducing the second optical information recording medium with the second light source, and when N-th order diffracted light coming from the circumference of a circle that is farthest from an optical axis on the area provided with the diffractive structure among the light flux from the first light source is converted into the light flux whose numerical aperture on the optical information recording medium side is NAH1, while when N-th order diffracted light coming from the circumference of a circle that is closest to an optical axis on the area provided with the diffractive structure among the light fluxes from the first light source is converted into the light flux whose numerical aperture on the optical information recording medium side is NAL1.
NAH1 less than NA1
(1/3) NA2 less than NAL1 less than NA2
When the objective lens of the optical pickup apparatus stated above is represented by an objective lens having the characteristics mentioned above, the same effect can be obtained, which is preferable.
Another embodiment of the invention will be described as follows. An objective lens of the optical pickup apparatus of the invention is characterized in that there is provided a lens surface on which an area provided with diffractive structure is provided on the outer circumference of an area that is not provided with diffractive structure up to the outermost circumference, and an area representing at least a part of the diffractive structure is in a form which makes at least two light fluxes each having a different wavelength (xcex1, xcex2) generate the maximum amount of diffracted light to be of the same order number (excluding 0 order). Incidentally, xe2x80x9cthe same order numberxe2x80x9d means that the order number is the same, including positive and negative signs. Further, xe2x80x9cthe outermost circumferencexe2x80x9d means the effective optical diameter of the lens surface. In the optical pickup apparatus which is actually equipped with an objective lens, however, xe2x80x9cthe outermost circumferencexe2x80x9d means the actually effective optical diameter through which a light flux from a light source enters, because the diffractive structure does not always need to be provided on an area on the outer circumferential side through which the light flux from a light source never enters at all.
The position where the area equipped with the above-mentioned diffractive structure is provided is determined by a wavelength of a light source used in an optical pickup apparatus that employs the objective lens stated above, a thickness of a transparent base board of an information recording medium, and density for recording information on the information recording medium, and is decided in accordance with the necessary numerical aperture of an objective lens determined for each of the information recording media.
For example, it is preferable that numerical apertures needed for objective lenses for reproducing or recording two information recording media in the optical pickup apparatus are different each other, and an area equipped with the diffractive structure is set in the vicinity of the smaller numerical aperture among the aforesaid numerical apertures. In virtue of the foregoing, it is possible to conduct light-converging for an appropriate spot diameter having the diffraction limit power for an information recording medium having a smaller necessary numerical aperture, and thereby to make the area that is not used usually for the information recording medium having a larger necessary numerical aperture, and might cause erroneous detection of focus signals by converging light through defocusing to be capable of contributing to image forming, on the contrary. Due to this,
it is possible to increase efficiency of using quantity of light from a light source that contributes to recording or reproducing of information, and to improve efficiency of an optical pickup apparatus by preventing erroneous detection of focus signals.
When an area that is not equipped with the diffractive structure is provided on the inner circumferential side of the area provided with a diffractive structure, the efficiency of using quantity of light is more improved, compared with the occasion where the diffractive structure is provided on the entire surface, and processing man-hour for a metal mold for forming lenses is shortened because the number of ring-shaped zones for diffraction is small, and fabrication cost for a metal mold can be reduced.
Further, it is preferable that the same order number stated above is one order. It is further preferable that the area provided with the diffractive structure is divided into an inner area and an outer area, and the inner area is made to be in a form which makes the same order number to be one order, and the outer area is made to be in a form which makes the same order number to be the order number of 2 order or more.
An embodiment of the objective lens for the optical pickup apparatus in the invention is characterized in that the optical pickup apparatus has a first light source having a wavelength of xcex1 and a second light source having a wavelength of xcex2 (xcex1 less than xcex2), the first light source emits a first light flux for reproducing or recording information for the first optical information recording medium having a transparent base board whose thickness is t1, the second light source emits a second light flux for reproducing or recording information for the second optical information recording medium having a transparent base board whose thickness is t2, the aforesaid objective lens has an area that is provided, on at least one surface, with diffractive structure that is rotation-symmetrical with respect to an optical axis, and the following expressions are satisfied when NA1 represents necessary numerical aperture of the objective lens on the optical information recording medium side that is needed for recording and/or reproducing the first optical information recording medium with the first light source, NA2 (NA2 less than NA1) represents necessary numerical aperture of the objective lens on the optical information recording medium side that is needed for recording and/or reproducing the second optical information recording medium with the second light source, the objective lens has on at least one surface thereof an area provided with a diffractive structure that is rotation-symmetrical with respect to an optical axis, N-th order diffracted light (N is an integer other than zero) coming from the area provided with the diffractive structure is utilized when recording and/or reproducing the first optical information recording medium with the first light source, M-th order diffracted light (M=N) coming from the area provided with the diffractive structure is utilized when recording and/or reproducing the second optical information recording medium with the second light source, and when the diffracted light with the order number generating the maximum amount of diffracted light coming from the circumference of a circle that is farthest from an optical axis on the area provided with the diffractive structure among the light flux from the first light source is converted into the light flux whose numerical aperture on the optical information recording medium side is NAH1, while when the diffracted light with the order number generating the maximum amount of diffracted light coming from the circumference of a circle that is closest to an optical axis on the area provided with the diffractive structure among the light flux from the first light source is converted into the light flux whose numerical aperture on the optical information recording medium side is NAL1.
NA1 less than NAH1
(1/3) NA2 less than NAL1 less than NA2
In the system using a light source having a wavelength of either one of 655xc2x130 nm for DVD and using a light source having a wavelength of either one of 785xc2x130 nm for DC in the optical pickup apparatus of a DVD/CD interchangeable type, 0.60xe2x89xa6NAH1 and 0.3xe2x89xa6NAL1  less than 0.45 are preferable.
In the range greater than the necessary numerical aperture NA2, it is preferable that spherical aberration for the second information recording medium is made to be flare. Due to this, it is possible to obtain an appropriate spot diameter for the second information recording medium side.
In the range greater than the necessary numerical aperture NA2, it is preferable that high-order diffracted light is used.
As a concrete design, in an optical pickup apparatus of a DVD/CD interchangeable type, an area equipped with a diffractive structure is provided on the numerical aperture smaller than the necessary numerical aperture for CD or more, and an area of the small numerical aperture or less is made to be a refracting interface that is not equipped with a diffractive structure. In the range from the small numerical aperture to the necessary numerical aperture for CD, spherical aberration is corrected for both DVD and CD, and for the necessary numerical aperture for CD or more, spherical aberration of DVD is corrected and is made to be flare so that a spot diameter may not be stopped down excessively for CD. Further, an area that is in the vicinity of the numerical aperture necessary for CD is made to be a basic aspheric surface in an aspheric surface shape that is different from an aspheric surface shape whose spherical aberration is corrected properly for DVD, then the diffractive structure is provided on that area against deterioration of spherical aberration caused for DVD under that basic aspheric surface alone, thus, a desired objective lens can be made by designing the diffractive structure so that the primary diffracted light by that area may contribute to image forming and spherical aberration may be corrected properly for DVD and primary diffracted light by that area may contribute to image forming. It is naturally possible to design an objective lens of the invention by various applications, without being limited to the aforesaid design. Incidentally, the optical pickup apparatus of a DVD/CD interchangeable type is an optical pickup apparatus wherein at least one of recording and reproducing of information is possible for at least one type of DVD, and at least one of recording and reproducing of information is possible for at least one type of CD. Various CDs include, for example, CD-R, CD-RW, CD-Video and CD-ROM, and DVDs include, for example, DVD-ROM, DVD-RAM, DVD-R and DVD-RW.
When an area provided with the diffractive structure is a diffractive surface having thereon ring-shaped zones for diffraction, and when xcfx86 (h) represents an optical path difference function of the diffractive surface (h represents a distance from an optical axis), it is possible to arrange so that dxcfx86(h)/dh is discontinuous or is substantially discontinuous function at the position of prescribed distance h. In the optical pickup apparatus for recording and reproducing information by an objective lens provided with ring-shaped zones for diffraction for the first and second information recording media each having a different thickness of a transparent base board, if a light flux on the outer side of prescribed numerical aperture is made to be flare under the state of use wherein the numerical aperture is on the smaller side, a beam diameter is not stopped down excessively even when the aperture restriction for a smaller numerical aperture is not used, and thereby, it is possible to obtain a relatively large spot diameter.
In this case, it is preferable that the ring-shaped zone which is i-th position when counted in the direction from an optical axis toward peripheral portion on the diffractive surface satisfies the following expression;
1.2xe2x89xa6pi+1/pixe2x89xa610
wherein, pi represents a width in the direction perpendicular to an optical axis of the i-th ring-shaped zone for diffraction counted in the direction from an optical axis toward the peripheral portion.
The preferable embodiment of the optical pickup apparatus of the invention is provided with a first light source that emits a first light flux having wavelength of xcex1 for reproducing or recording of information for the first optical information recording medium with a transparent base board whose thickness is t1, a second light source that emits a second light flux having wavelength of xcex2 (xcex1 less than xcex2) for reproducing or recording of information for the second optical information recording medium with a transparent base board whose thickness is t2, an optical detector that detects light from the first and second information recording media, and with an objective lens wherein there is provided a lens surface on which an area provided with a diffractive structure is provided on the outer circumference side of the area provided with a diffractive structure up to the outermost portion, and at least a part of the area of the diffractive structure is in a form which makes at least two light fluxes each having a different wavelength (xcex1, xcex2) generate the maximum amount of diffracted light to be of the same order number (excluding 0 order).
Another preferable embodiment of the optical pickup apparatus in the invention is characterized in that there are provided a first light source for emitting a first light flux having wavelength of xcex1 for reproducing or recording information for the first optical information recording medium whose transparent base board has a thickness of t1, a second light source for emitting a second light flux having wavelength of xcex2 (xcex1 less than xcex2) for reproducing or recording information for the second optical information recording medium whose transparent base board has a thickness of t2, an optical detector that detects light from the first and second information recording media, and an objective lens wherein an area provided with a diffractive structure that is rotation-symmetrical with respect to an optical axis is provided on the outer circumference of an area provided with no diffractive structure up to the outermost portion on at least one surface of the objective lens, and the following expressions are satisfied when NA1 represents necessary numerical aperture of the objective lens on the optical information recording medium side that is needed for recording and/or reproducing the first optical information recording medium with the first light source, NA2 (NA2 less than NA1) represents necessary numerical aperture of the objective lens on the optical information recording medium side that is needed for recording and/or reproducing the second optical information recording medium with the second light source, the objective lens has on at least one surface thereof an area provided with a diffractive structure that is rotation-symmetrical with respect to an optical axis, N-th order diffracted light (N is an integer other than zero) coming from at least a part of the area provided with the diffractive structure is utilized when recording and/or reproducing the first optical information recording medium with the first light source, M-th order diffracted light (M=N) coming from at least a part of the area provided with the diffractive structure is utilized when recording and/or reproducing the second optical information recording medium with the second light source, and when the diffracted light with the order number generating the maximum amount of diffracted light coming from the circumference of a circle that is farthest from an optical axis on the area provided with the diffractive structure among the light fluxes from the first light source is converted into the light flux whose numerical aperture on the optical information recording medium side is NAH1, while when the diffracted light with the order number generating the maximum amount of diffracted light coming from the circumference of a circle that is closest to an optical axis on the area provided with the diffractive structure among the light fluxes from the first light source is converted into the light flux whose numerical aperture on the optical information recording medium side is NAL1.
NA1xe2x89xa6NAH1
(1/3) NA2 less than NAL1 less than NA2
Incidentally, in the invention, the first and second optical information recording media include, for example, various types of CD such as CD, CD-R, CD-RW, CD-Video and CD-ROM and various types of DVD such as DVD, DVD-ROM, DVD-RAM, DVD-R and DVD-RW, or a disk-shaped information recording medium such as MD, and further, high density information recording media wherein recording density has been enhanced are also included.